General Information 1

General Information 1 1.1 CLEANING F VLUMETRIC GLASSWARE All the volumetric glassware (Burette, Pipette, Volumetric flasks etc) must be perfectly clean, free from dust and greasy impurities. Unreliable results are liable to be obtained with dirty apparatus. The cleanliness of a glass vessel can easily be tested by filling it with distilled water and then pouring it out. If an unbroken film of water remains on the walls, the vessel is clean; the formation of droplets indicates the presence of impurities and the vessel needs cleaning. For cleaning the glassware, firstly soak the apparatus in 10% of soap solution for 15 20 minutes. Wash it with tape water, then with HCl and finally with distilled water. If vessels are not cleaned by this method then soak the vessel in cleaning mixture (equal volume of concentrated and saturated solution of K 2 Cr 2 7 ) for 1 hour. Pour the mixture and wash thoroughly with tap and distilled water. 1.2 STRENGTH F SLUTIN It is expressed in following ways: (i) Percentage: Number of gms of the substances dissolved in 100 gms of its solution. It is generally used in dilute solutions of electrolytes in water. (ii) Molality: Number of gm moles of substances dissolved in 1000 gms of solvent. (iii) Molarity: Number of gm moles of the substances is dissolved per liter of the solution. If one gm mole of a substances is dissolved in a solvent and the solution is made up to one liter mark of the flask, such solution is called Molar Solution. If w gms of a substance is dissolved in V.c.c. of the solution and Mol. of the substance is M, w 1000 Molarity M V gm/ litre molwt.. 1

2 Practical Physical Chemistry (iv) Normality: Number of gm equivalents of the substance dissolved per liter of the solution. If one gm equivalent of an acid, base or salt is dissolved in water and the solution is made up to one liter, such solution is called Normal Solution. If w gms of a substance is dissolved in V.c.c. of the solution and equivalent weight of the substance is E, w Normality E 1000 V gm/ litre Eq. 1.3 STANDARD SLUTIN A solution of known strength is called Standard Solution. It is prepared either by direct weighing of the substance or after titrating it against another standard solution i.e., standardisation with the help of another standard solution. Calculation of Normality of prepared solution (i) Wt. of the substance + Weighing tube x gms. (ii) Wt. of empty weighing tube y gms. (iii) Eq. weight of the substance E. In 100 c.c. measuring flask, the normality of the solution 1000( x- y) 10( x- y) 100 E E 10 Weight taken Eq. Calculation of the desired weight for preparing of standard solution Suppose equivalent weight of the substance is E, and we have to prepare a solution of x normality in V ml measuring flask. Substance in gm per liter x E Substance in gm in V ml 1.4 TITRATIN x E V. 1000 It is the process for standardisation of a solution. Two solutions are used in every titration where strength of one is generally known. The solution to be pipetted is taken in a conical flask by using pipette of 10 or 20 c.c. and the other solution is taken in the burette. Indicator is added if necessary at this stage of the solution taken in the conical flask and burette solution is generally poured into it. A sharp colour change occurs at the completion of the reaction. This stage is called End Point. Equivalent system is use in the calculations of volumetric results. Such calculations are rendered very simple because at the end point in a titration, the number of equivalent of the substance titrated is equal to the number of equivalents of the standard solution employed. Thus, if the volumes of

General Information 3 solutions of two substances A and B with normality N A and N B which exactly react with one another are V A and V B cm 3 respectively, then these volumes contain the same number of milligram equivalents of A and B. Thus, at the equivalence point: Number of mg equivalent of A Number of mg equivalent of B But number of mg equivalent of A N A V A and number of mg equivalent of B N B V B Therefore at the equivalence point: N A V A N B V B This is known as normality equation. When any three quantities in this equation are known, the fourth may be readily calculated. 1.5 EQUIVALENT WEIGHT FR VARIUS VLUMETRIC REACTINS Equivalent weight varies with the type of reaction, and since it is difficult to give a clear definition of equivalent weight that may cover all the reactions. Sometimes, a compound possesses different equivalent weights in different chemical reactions. Thus the equivalent weight of a substance can be determined only after considering the reaction in which it is participating and this is done as described here: (i) Neutralization Reactions (a) Equivalent weight of Acids: Molecular weight ofacid Equivalent of an acid Number of replaceable hydrogenatoms or Mol. wtofacid. Basicity of acid Monobasic acids are: HC1, HBr, HI, HN, CH 3 CH, etc. Dibasic acids are:, C 2, etc. Tribasic acids are: H 3 P and H 3 B, etc. So equivalent of HC1 Equivalent of (b) Equivalent weight of Bases: Equivalent of base 36. 56 36.56 1 98 49.0 2 Molecularweightof base Number of replaceableofhydroxylgroups Mol. ofthe base Acidityofthe base Monoacid bases are: NaH, KH, etc. Diacid bases are: Ba(H) 2. 8, Ca(H) 2 etc. So, equivalent of NaH 40 40 1

4 Practical Physical Chemistry 315 Equivalent of Ba(H) 2. 8 157.5 2 (ii) Redox Reactions (a) Equivalent weight of an oxidising agent: Its equivalent weight depend upon the amount of oxygen liberates for a reaction. Potassium permanganate (KMn ): It acts as an oxidising agent in acidic, neutral and alkaline mediums. The amount of oxygen made available for oxidation is different for different mediums and hence equivalent weight varies with the nature of the medium. Acid medium: 2KMn + 3 K 2 + 2Mn + 3 + 5[] From this equation, it follows that: 2KMn 5[] 2 158 5 16 (Molecular weight of KMn 158) 2 158 5 8 2 or 158 5 8 i.e., 8.0 gm of oxygen will be made available by 158 31.6 gm weight of KMn4 5 Equivalent of KMn in acid medium 31.6 Neutral medium: 2KMn + 2Mn 2 + 2KH + 3[] From this equation, it follow that: 2KMn 3[] or 2 158 3 16 2 158 3 2 8 or 158 3 8 i.e., 8.0 gm of oxygen will be made available by 3 158 52.66 gm weight of KMn4 Equivalent of KMn in neutral medium 52.66 Alkaline medium: 2KMn + 2KH 2K 2 Mn + + [] From this equation it follows that: 2KMn [] or 2 158 2 8 158 8 i.e., 8.0 gm oxygen will be made available by 158 gm weight of KMn Equivalent of KMn in alkaline medium 158 (b) Equivalent weight of a reducing agents: Ferrous ammonium sulphate [Fe.(NH 4 ) 2.6 ] (F.A.S.) In acidic medium: 2Fe + + Fe 2 ( ) 3 +

General Information 5 From this equation it follows that: 2 moles of ferrous salt [] 2 Fe.(NH 4 ) 2.6 [] (Mol. of FAS 392.0) 2 392 2 8 392 8 i.e., 8.0 gm oxygen will combine with 392.0 gm of F.A.S. Eq. of F.A.S. 392.0 It may be noted that in writing chemical reactions Fe is only shown, since in F.A.S. it is the only constituent, (NH 4 ) 2 is not active in redox reaction and hence does not take part in chemical reaction. The equivalent weights of some important acids and alkalies have been collected in the following Table 1.1: Table 1.1 Acid/Base Mol. Basicity/ Acidity Eq. Acid/Base Mol. Basicity/ Acidity Eq. HCl 36.5 1 36.5 Ca(H) 2 74 2 37 HN 63 1 63 Na 2 C 106 2 53 CH 3 CH 60 1 60 K 2 C 138 2 69 98 2 49 CaC 100 2 50 C 2.2 126 2 63 NaHC 84 1 84 H 3 P 98 3 32.66 KHC 100 1 100 NaH 40 1 40 C CH C CH KH 56 1 56 KHC 2. C 2.2 118 2 59 254 3 84.76 Ba(H) 2.8 15 2 157.6 Na 2 B 4 7.10 81.4 2 190.7

6 Practical Physical Chemistry Table 1.2. Equivalent Weights of xidising and Reducing Agents by Ionic Equations Substance Mol. Partial Ionic equation No. of Electrons Gained/Lost per Molecule Eq. xidants KMn (acid) 158 Mn 4 + 8H+ + 5e Mn 2+ + 4 5e Mol. /5 KMn (neutral) 158 Mn 4 + 2 + 3e Mn 2 + 4H 3e Mol. /3 KMn (basic) 158 Mn 4 + e Mn 2 4 1e Mol. K 2 Cr 2 7 (acid) 294.2 Cr 2 2 7 + 14H+ + 6e 2Cr 3+ + 7 6e Mol. /6 2 (acid) 34 2 + 2H + + 2e 2 2e Mol. /2 Mn 2 (acid) 87 Mn 2 + 4H + + 2e Mn 2+ + 2 2e Mol./2 HN (conc.) 63 N 3 + 2H+ + e N 2 + 1e Mol. HN (dil.) 63 N + 4H + + 3e N + 2 e Mol. /3 Cu. 5 (neutral) 249.5 2Cu 2+ + 4I + 2e Cu 2 I 2 + 2I 1e Mol. I 2 254 I 2 + 2e 2I 2e Mol./2 KI 214 I 3 + 6H + + 6e I + 3 6e Mol. /6 Reductants As 2 198 2As 3+ 2As 5+ + 4e 4e Mol./4 Fe. (NH 4 ) 2.6 392 Fe 2+ Fe 3+ +e 1e Mol. FeC 2.2 180 Fe 2+ + C 2 2 4 Fe 3+ + 2C 2 + 3e 3e Mol./3 Fe.7 278 Fe 2+ Fe 3+ + e 1e Mol. C 2.2 126 C 2 2 2 2C 2 + 2e 2e Mol./2 Na 2.5 248 2 2 3 S 4 2 6 + 2e 1e Mol.

General Information 7 1.6 INDICATRS The reagent used to locate the exact completion stage i.e., end point of the reaction by showing change in its colour is called an indicator. n adding even the smallest excess of the titrant, beyond what is necessary for exact completion of the reaction, the indicator changes colour. The following are certain types of indicators which will be deal when considering different divisions of volumetric analysis. (a) External indicator: It is placed outside on a reference porcelain plate, e.g., K 3 Fe(CN) 6 dilute solution in the titration of ferrous sulphate or ferrous ammonium sulphate against K 2 Cr 2 7. (b) Self indicator: When colour of one of the titrants acts as indicator, e.g., pink colour of permanganate ion in the titration of oxalic acid or ferrous ions against KMn. (c) Internal indicator: It is added to solution taken in the conical flask. The most common internal indicators are: (i) Phenolphthalein (ii) Methyl orange (iii) Starch solution (iv) Methyl red (v) N-phenyl anthranilic acid Table 1.3. Common Acid-base Indicators No. Indicator pk ln ph Range In Acid Soln. Colour In Alkaline Soln. 1. Methyl orange 3.7 3.1 4.4 Red Yellow 2. Methyl red 5.1 4.3 6.1 Red Yellow 3. Litmus 6.5 5.5 7.5 Red Blue 4. Phenol red 7.8 6.8 8.4 Yellow Red 5. Phenolphthalein 9.7 8.3 10.0 Colourless Red 1.7 IDINE TITRATIN The redox-titration using iodine directly or indirectly as an oxidising agent are called Iodine titrations. These are of two types: 1. Iodimetric titrations: Iodimetric titrations are defined as those iodine titrations in which a standard iodine solution is used as an oxidant and iodine is directly titrated with a reducing agent. Iodimetric procedures are used for determination of reducing agents like thiosulphates, sulphites, arsenites etc by titrating them against standard solution of iodine run in from a burette. xidationreduction reactions are: Na 2 + I 2 Na 2 S 4 6 + 2NaI

8 Practical Physical Chemistry 2. Iodometric titrations: Iodometric titrations are defined as those iodine titrations in which some oxidising agent liberates iodine from an iodide and then liberated iodine is titrated against standard solution of a reducing agent added from a burette. In such titrations a neutral or an acidic solution of oxidising agent is employed. The amount of iodine liberated from an iodide (i.e. KI) is equivalent to the quantity of the oxidising agent present. Such as 2 Cu + 4KI Cu 2 I 2 + 2K 2 + I 2 Liberated iodine in the above reaction is titrated against standard sodium thiosulphate. (i) Equivalent weight of reducing agents: 2Na 2 + I 2 Na 2 S 4 6 + 2NaI Sodium thiosulphate (hypo): 2Na 2 + I 2 Na 2 S 4 6 + 2NaI 2Na 2 2I 2 m.w. 2 127 Parts of thiosulphate reacting with 127 parts by of iodine 2 mw.. 127 2 127 Equivalent weight of Na 2 m.w. 158.1 Equivalent weight of Na 2.5 m.w. 248.1. (ii) Equivalent weight of oxidising agents: Copper sulphate, Cu.5 2Cu + 4KI 2CuI + I 2 + K 2 2 Cu. 5 I 2 2 m.w. 2 127 Equivalent weight of hydrated copper sulphate m.w. 249.67. (iii) Indicator and end point detection: Iodine produces with starch solution an intensely blue iodo complex. In all iodine titrations freshly prepared 1% starch solution acts as an indicator. In iodimetric titrations, this starch solution is added to the solution of reducing-agent taken in the conical flask and then oxidising agent is added from the burette. The end point is indicated by the appearance of blue colour. In the iodometric titrations the iodide (e.g. KI) solution and the oxidising agent are taken into a conical flask. The standard solution of reducing agent, e.g., sodium thiosulphate is run from burette. When the yellowish brown colour is faint, the starch is added. It produces a deep blue complex. Now the thiosulphate solution is added drop by drop with proper shaking. The end point will be indicated when blue colour just disappears.

General Information 9 Table 1.4. Data on the Specific Gravity, Percentage Composition, Normality etc. of Some Concentrated Reagents Reagent Specific gravity Percentage by weights Approximate normality Weight of anhydrous reagent (gm) in c.c. 1.84 96 36N 1.80 HCl 1.19 37.9 12N 0.366 HN 1.42 69.8 16N 0.991 NH 4 H 0.882 34.95 15N 0.308 Glacial CH 3 CH 1.055 99.5 17N 1.80 Table 1.5. Concentration of Aqueous Solution of Common Acids and Ammonia Reagents Molarity of Concentrated solution Normality of concentrated solution Vol. required to make 1dm 3 0.1N (approx. cm 2 ) Hydrochloric acid 11.6 11.6 8.6 Nitric acid 15.4 15.4 6.5 Sulphuric acid 17.8 35.6 2.8 Phosphoric acid 14.6 43.8 2.3 Acetic acid 17.4 17.4 5.8 Ammonia 14.8 14.8 8.6 qqq